Radio review:When a picture is better heard than seen

What's the true story? Well, the woman's obviously pregnant ... the man in the hat might be a vicar ... they both look miserable ... he's got a wicked, wicked face - those flaring nostrils - but she could be a surrogate bride ... what about the dog? And the people in the mirror ... What Are They Looking At? (R3).

At a picture, painted at Bruges in 1434 on oak, in oils. It belonged to Margaret of Austria, then to Mary of Hungary, then it disappeared: it turned up, covered in dust, in Brussels after Waterloo. It was brought to England and sold to the National Gallery for pounds 630 in 1842. It is Jan Van Eyck's seductive, mesmerising masterpiece, The Arnolfini Marriage, and it is being given the Plowright treatment.

If ever you have switched on the radio casually, while peeling carrots, and had your attention hijacked as the saucepan boils dry and your family howls in vain for supper; if you have discovered a hitherto unimagined but suddenly all-consuming interest in Japanese insects, Grinling Gibbons's carving, JM Synge's music or the problems Erasmus faced travelling through a storm; if you have found yourself saying "Yes, that's why I love the radio better than a month at the seaside" - then the chances are you've been listening to a production by Piers Plowright.

Here's a trade secret: radio reviewers receive advance tapes of some programmes. They accumulate faster than dust and after a week or two you'd almost pay to be shot of them. But some you want to keep forever, to hear again in peace, when nobody wants you to write about them. And guess what, most of my stack of those programmes have been produced by this man. The bad news is that, after 30 years with the BBC, he is retiring. We shall hear one more new production, on Radio 4 next month; there will be repeats and, with luck, he will be cajoled back occasionally as a freelance, but this was his last bona fide Radio 3 programme and it was wonderful.

His gift is to take a subject and turn it slowly in the light, in a leisurely, fascinated, careful and gently humorous way. His enthusiasm underpins the whole endeavour but he lets you catch it gradually. By the end, having heard the pompous pronouncements of art historians, the hilarious comments of people wandering past, the reverence of the veteran visitor, the excitement of a milliner who copied the hat for a shop and did well out of it, the awe of a blind woman feeling it - and the chuckling, reminiscent, imagined voice of the jobbing artist himself - you want to drop everything and rush off to the Sainsbury Wing just to have another look at that picture, now that you know how to.

We can only hope that whoever is in charge of such things does in indeed cajole Plowright back, and often.

Happily, other people make fine programmes too, and this was a particularly rich week. Shadowlands (R4) hit the radio, with Michael Williams as a singularly touching CS Lewis. Subtly and unsoppily directed by Elijah Moshinsky, William Nicholson's stage play settled into the medium as if created for it. My only problem was that I began to hear Zoe Wanamaker (as Joy Gresham) reading her script, albeit with an impeccable New York accent, right up to the moment when she thought she was dying and, at last, dropped the paper, picked up some spontaneity and got real.

The reality, or not, of the story was discussed in Shadowlands: Truth and Fiction (R4). Presented by Humphrey Carpenter (who proved to be the son of the Bishop of Oxford who refused to marry Lewis to the divorced Joy), it turned into a fight. On one side, the intolerably lofty AN Wilson described Lewis as beset by vanity and a desire to play God (he should talk), and Joy as a foul-mouthed woman, "smoking her head off" (as if that was a crime in the Fifties) and determined to use any trick to marry him. On the other, Joy's son Douglas Gresham, quietly replied, "My mother might have overdone it a bit by getting cancer and dying," and paid tribute to Lewis as a devoted and beloved step-father. Nicholson's dramatisation does not tell the whole truth - any more than Shakespeare's histories do - but I trusted Douglas, and he trusts the play.

In Northern Ireland, the whole truth is rarer than a white truffle and harder to dig up. In North and South Travels along the Irish Border (R4) Colm Tibn wisely decided to grub about for little bits of it and present his finds without garnish. He discovered a Derry bookseller lamenting the post-ceasefire disappearance of "radical tourists"; a thoughtful Strabane man who remembered soldiers rescuing his wee dog from a flood; an ancient stone circle - brutal, plain and nothing whatever to do with art. And he collided with the cult of "heritage" - which takes the harm out of history while making money from it. They should listen to Tibn at Stormont, and take a deep breath.

Mel and Michael McDonagh, Irish travelling people, sang the serendipity of wanderlust in The Return of Itchy Feet (R4) - so beguilingly that you'd want to be off with them. "If you had a horse," they said, "you could travel the length and breadth of Ireland for nothing, only the grass by the road".

And if you had Molly, you'd have a friend. Dylan Winter told us about Molly's Last Ride (R4), along the ancient Ridgeway. Only, er, nothing happened. Perhaps he dropped his tape-recorder ... Anyway, he filled the time with a Molly retrospective: finest moments in the life of a Welsh cob, as strong and old as a tax-exempt Land Rover. We revisited Molly on the Oregon trail and Molly pulling a canal-barge full of soap; we sighed with gusty nostalgia.

But Molly's not bound for the knacker's - nor even for retirement, she has a date with a stallion in Northamptonshire and Winter nurses high hopes for the spring. You couldn't do that with an old Land Rover, and that's the truth.

Piers Plowright's 'Nine Fruit Trees' will be broadcast on 29 Oct at 8.30pm, on R4.